The present invention is related to polishing tools, particularly to non-contact polishing tools, and more particularly to a non-contact polishing tool having adjustable removal footprint geometry by the use of a plurality of orthogonal slurry flow geometries.
Technology for manufacturing surfaces to precision tolerances is critical in fields such as inertial confinement fusion (ICF), x-ray lithography, camera lenses, and various other optical devices. It is estimated that the world-wide market for such optical devices will be in the billions of dollars. Also, the availability of highly accurate aspheric optics is a key to future reductions in semiconductor line widths via projection lithography.
Several new optical figuring technologies are being developed throughout the world to eliminate the poor repeatability and high cost associated with traditional pitch polishing. Ion beam figuring (IBF) and plasma-assisted chemical etching (PACE) both have controllable removal footprints that may be applicable for high accuracy figuring, but both require expensive vacuum systems and are applicable only to limited sets of materials. For example, IBF has been very successful during the final figure corrections of the Keck Telescope segments. Ductile-mode grinding shows promise as a deterministic shaping process for producing smooth damage-free surfaces, but it has not yet been demonstrated to produce highly accurate aspheric surfaces that do not require post-polishing, particularly in fused silica. Stressed-lap and stressed-part lapping are currently being used with good success for figuring large telescope optics, but have not been applied to the much smaller optics, especially with respect to the tolerances and spatial wavelengths of relevance to lithographic optics.
Elastic emission machining (EEM), see Y. Mori et al., "Mechanism of Atomic Removal in Elastic Emission Machining", Precision Engineering, January 1988, Vol. 10, No. 1, pp 24-28; flow polishing, see P.C. Baker, "Advanced Flow-Polishing of Exotic Optical Materials", X-Ray/EUV Optics for Astronomy and Microscopy, SPIE, Vol. 1160, 263-270 (1989); and float polishing, see J. M. Bennett et al., "Float Polishing of Optical Materials", Applied Optics, 26(4), 696-703 (1987), are all non-contact polishing techniques that may produce minimal subsurface damage. They all utilize the same fundamental material removal process: a fluid dynamic flow field is established to carry a fine abrasive slurry to the optical surface which transports away material by a sufficiently gentle transport mechanism that does not disrupt the structure of the surface layers. In one form or another, these processes are currently being used to prepare aspheric surfaces. An EEM approach is being developed to figure aspheric surfaces in support of the Advanced Processing and Machining Technology Research Association (AAMTRA), see the "Advanced Material Processing & Machining: Unveiling the Technology of the 21st Century", AAMTRA brochure describing the consortium's approach for soft x-ray lithography, members include Cannon, Toshiba, Nikon, Hitachi, etc., c. 1991.
While the non-contact polishing techniques of the above-referenced optical finishing strategies, have strengths and weaknesses for aspheric optics, there is a need for a non-contact, polishing approach which utilizes the strengths of these prior strategies, but eliminates the weakness thereof. This need is satisfied by the present invention which combines two orthogonal slurry flow geometries to provide flexibility in altering the shape of the removal footprint. The invention provides a non-contact polishing tool that will meet stringent shape (figure) and finish (roughness) tolerances on precision surfaces during their fabrication. The tool is particularly useful for surfaces that have very tight geometrical shape tolerances.